Method for preserving the effectiveness of a metal cleaning bath



Patented Jan. 11, 1949 METHOD FOR PRESERVING THE EFFEC- TIVENESS OF AMETAL CLEANING BATH Hugh G. Webster, Detroit, Mich.

No Drawing; Application May 10, 1946,

, Serial No. 668,777

1 Claim. 1

This application relates to methods for preserving the effectiveness ofmetal cleaning baths, particularly baths of the molten alkali salt typeuseful in metal cleaning.

Users of such baths have found it necessary from time to time to disposeof the bath after it has been in use for some while, and replace it by afresh bath, because, for reasons hitherto not known, the efiectivenessof the bath, which deteriorates as the bath is used, has dropped below apoint of minimum usefulness.

I have discovered that the deterioration of the bath is due to theformation of excessive amounts of alkali salt carbonate in the bath asthe bath is used. Such carbonates are salts of the metal alkali whosesalts form the bath. The carbonate may be formed either by the effect ofatmospheric conditions or by chemical reactions arising during the metalcleaning. In such reactions some of the salts, which may be hydroxides,chlorides, nitrates, aluminates, fluorides, etc., are transformed intocarbonates. I have discovered that as the percentage of carbonateincreases in a bath, the efieotiveness decreases.

I have also developed a method for removing the carbonate from a bathand thus removing the cause of the deterioration of the bath and thismethod I will now describe. A molten bath of the character underconsideration, including molten sodium hydroxide, molten sodium nitrate,molten sodium chloride, etc., operates at a temperature considerablyabove its melting point; at such temperature, generally 850-900 F., alarge amount of carbonate is soluble in the bath. The first step inremoving the carbonate, therefore, is to reduce the temperature of thebath until the temperature drops to a point very close to or even belowthe bath solidfication point, approximately 600 F. Thereafter, the bathis reheated and brought up to the melting point or slightly above untilit begins to become fluid. Temperatures in the range of 650 F. to 700 F.have been found sufficient for this purpose, it being merely requiredthat the temperature be very slightly above the melting point of thebath, whatever that melting point is.

At this point, the carbonate will be found to have precipitated out ofthe bath and drop to the bottom, obviously due to the fact that less ofthe carbonate is soluble in the lower temperature bath than at theoperating temperature of the bath, considerably higher, the drop fromthe operating temperature, around 850 F., to the melting point, 600 F.,being sufficient to cause a very large amount of the carbonate to dropout of the bath as a precipitate.

In the bottom of the tub in which the bath is confined is a sludge paninto which the precipitated carbonate drops. Very quickly after the bathhas been brought up to the melting point and the carbonate has beendropped from the bath, that is, very quickly after the sludge hasformed, the sludge pan is lifted up and out of the bath, thus removing asubstantial portion of the carbonate from the bath.

With the carbonate removed, the bath is r turned to operatingtemperature and its effectiveness is no longer impaired by the presenceof excess carbonate.

In some cases, it may be necessary to replenish the supply of othersalts to the bath after the carbonate is removed and this of course, isa simple matter. With the supply of other salts replenished, the bath isonce again effective for use.

Now having described the method herein disclosed for preserving theeffectiveness of a metal cleaning bath of the molten alkali salt type,reference should be had to the claim which follows.

I claim:

A method for preserving the effectiveness of a metal cleaning moltenalkali salt bath of the type wherein an excess of alkali salt carbonateis formed, either by atmospheric effects, or by chemical reactionsarising during metal cleaning, which comprises removal of the carbonatefrom time to time as by precipitating it out of the bath and thenremoving it as a precipitate as follows:

reducing the temperature of the bath to the solidification point, thenincreasing the temperature of the bath to the melting point, thuscausing the carbonate, less soluble in the bath at the melting pointthan at higher temperatures, to precipitate out of the bath, and drop tothe bottom, and quickly removing the carbonate precipitate.

HUGH G. WEBSTER.

No references cited.

